[Sidenote: _Horizon._]
12. The _Horizon_ is that great circle which divides the upper, or
visible Hemisphere of the world, from the lower, or invisible: This
circle is distinguished into two sorts, the _Sensible_, and the
_Rational_.
[Sidenote: _Sensible Horizon._]
The _Sensible_, or _Apparent Horizon_, is that circle which limits or
determinates our prospect, whether we are at land or sea, reaching as
far as we can see, or it is that circle where the Sky and the Earth, or
Water, seem to meet. When we are on _Terra Firma_, this circle commonly
seems rugged and irregular, occasioned by the unevenness of the ground
terminating our prospect; but at sea there are no such irregularities;
the semidiameter of this circle varieth according to the height of
the eye of the observer; if a man of six feet high stood upon a large
plain, or the surface of the sea, he could not see above three miles
round.
This circle determines the rising and setting of the Heavenly bodies,
and distinguishes Day and Night.
[Sidenote: _Rational Horizon._]
The _Rational_, or true _Horizon_, is a great circle passing thro’ the
center of the Earth, parallel to the sensible Horizon, being distant
from it by the Earth’s semidiameter, which is about 3980 miles: This
distance is nothing in comparison of the immense distance of the Sun
and the fixed Stars, therefore astronomers make no distinction between
these two circles, but consider the apparent Horizon, or that wherein
the Sun appears to rise and set, as passing thro’ the center of the
Earth.
[Sidenote: _Cardinal Points of the Horizon._]
This circle is divided by astronomers into four quadrants, and each of
the quadrants into 90 degrees, _&c._ The four points quartering this
circle are called the _Cardinal Points_, and are termed the _East_,
_West_, _North_, and _South_. The _East_ is that point of the Horizon
where the Sun rises when he is in the equinoctial, or on that day when
he ascends above the Horizon exactly at six o’clock; and the _West_
is that point of the Horizon which is directly opposite to the East,
or where the Sun Sets when he is in the Equinoctial. The _South_ is
90 degrees distant from the East and West, and is toward that part of
the Heavens wherein the Sun always appears to us in _Great-Britain_ at
Noon; and the _North_ is that part of the Heavens which is directly
opposite to the South: Or, the North and South points of the Heavens
may be found by turning yourself either directly towards the East or
the West: If you look towards the (East/West) the (South/North) will be
to the right Hand, and the (North/South) to the left.
[Sidenote: _Points of the Compass._]
Besides the aforementioned divisions of the Horizon into degrees.
_Mariners_ divide it into 32 equal parts, which they call the _Points
of the Compass_; to each of which points they give a particular name,
compounded of the four Cardinals, according to what quarter of the
Compass is intended.
[Sidenote: _Zenith._]
[Sidenote: _Nadir._]
The center of the Horizon is the place of observation, and the Poles of
it are one exactly over our heads, called the _Zenith_; and the other
exactly under our feet, called the _Nadir_.
[Sidenote: _Vertical Circles._]
[Sidenote: _Meridian._]
[Sidenote: _Azimuth._]
13. All circles conceived to pass thro’ the Zenith and Nadir, are
called _Vertical Circles_, or _Azimuths_. Of these circles, that which
passeth thro’ the North and South points of the Horizon, is called the
_Meridian_; so that when any object is upon the Meridian, it then bears
either due South, or due North from us; and the _Azimuth_ of any object
is an arch of the Horizon intercepted between the vertical circle
passing through it, and either the North or South part of the Meridian;
which part is commonly specified.
The meridian passes thro’ the Poles of the world, as well as through
the Zenith and Nadir, and therefore is a secundary both of the
equinoctial and the horizon: This circle divides the globe into the
_Eastern_ and _Western Hemispheres_, and the Poles of it are the _East_
and _West_ points of the _Horizon_. All the heavenly objects are,
during one half of their continuance above the horizon, in the Eastern
Hemisphere, and for the other half in the Western; so that whenever the
Sun arrives upon the upper part of the meridian, it is then _Noon_, or
_Mid-day_, which is the reason why this circle is called the meridian;
and when he comes to the lower part, it is then _Midnight_.
[Sidenote: _Prime Vertical._]
The vertical circle passing thro’ the East and West points of the
horizon, is called the _Prime Vertical_, or _Circle_ of _East_ and
_West_, so that when any object is upon this circle in the Eastern
hemisphere, it appears due East; and if it be in the Western
hemisphere, it appears due West.
[Sidenote: _Amplitude._]
That degree in the horizon wherein any object rises or sets from the
East or West points, is called the _Amplitude_; which for rising is
called _Amplitude Ortive_, and _Occasive_ for setting; which must be
also denominated whether it be Northerly or Southerly.
It may be observed, that the _Amplitude_ and _Azimuth_ are much the
same; the amplitude shewing the bearing of any object when he rises or
sets, from the East or West points of the horizon; and the azimuth,
the bearing of any object when it is above the horizon, either from
the North or South point thereof. As for example, if an object rises or
sets within 10 degrees of the East or West, suppose towards the South,
we accordingly say, its _Amplitude_ is 10 degrees Southerly; but if
an object, that is of any height above the horizon, should be in the
vertical circle, passing thro’ the before-mentioned point, we then say,
its _Azimuth_ is 80 degrees from the South, or 100 degrees from the
North, both which expressions signify the same.
[Sidenote: _Almacanthers._]
[Sidenote: _Altitudes._]
[Sidenote: _Meridian Altitude._]
[Sidenote: _Zenith Distance._]
14. All circles drawn parallel to the horizon, in the upper hemisphere,
are called _Almacanthers_, or _Parallels of Altitude_: So that the
_Altitude_ of any point in the Heavens is an arch of the vertical
circle passing thro’ that point, and intercepted betwixt it and the
horizon; and if the object be upon the meridian, it is commonly called
the _Meridian Altitude_. The complement of the altitude, or what it
wants of 90 degrees, is called the _Zenith Distance_.
The horizon (by which we mean the rational) is represented by the
upper surface of the Wooden frame, wherein the globes are placed; upon
this horizon are described several concentric circles, the innermost
of which is divided into degrees, which ought to be numbered both ways
from the East and the West, until they end at 90 degrees in the North
and South points. The use of these divisions is to shew the amplitudes
of the Sun and Stars, at their rising and setting: Also in some
convenient place upon this horizon, there is commonly noted the points
of the Compass. Without the before-mentioned circle there is drawn the
ecliptic with its divisions, into signs, and degrees, and a circle of
months and days: The use of these two circles is to serve as a kalendar
to shew the Sun’s place at any time of the year, and by that means to
find his place in the _Ecliptic_, drawn upon the globe itself.
The _Vertical Circles_, and the _Parallels of Altitude_, are supplied
by a thin plate of brass, having a nut and screw at one end to fasten
it to the brass meridian in the Zenith point; which being done, the
lower end of it may be put between the globe it self, and the inner
edge of the horizon, and so turned round about to any point required.
[Sidenote: _Quadrant of Altitude._]
The fiducial edge thereof representing the _Vertical Circles_, and the
_Degrees_ upon it, describing the Parallels of Altitude. This thin
plate is called the _Quadrant of Altitude_.
The center of the horizon being the place of observation, it is
evident that this circle, and all the others belonging to it, are
continually changed, which way soever we move; wherefore we may
suppose the horizon, with its secundaries and parallels, to invest
the globe like a _rete_ or net; and to be moveable every way round
it. This is very naturally illustrated by the globes; if we move
directly North, or directly South, the change made in the horizon,
is represented by moving the brass meridian (keeping the globe from
turning about its axis) in the notches made in the wooden horizon,
just so much as we travelled. If our course should be due East, or
due West, the alterations made thereby are represented by turning the
globe accordingly about its axis, the brass meridian being kept fixed;
and if we steer betwixt the meridian and the East or West points,
then we are to turn the brass meridian, and also the globe about its
axis accordingly; the sum of which is, let the spectator be at what
point soever of the Earth’s surface, he’ll there gravitate, or tend
exactly towards its center, and imagine himself to be on the highest
part thereof, (the unevenness of the ground not being here considered)
wherefore if we turn the globe in such a manner as to bring the several
progressive steps of a traveller successively to the Zenith, we shall
then have the successive alterations made in the horizon, in every part
of his journey. This explication being well considered, will be of help
to young beginners, to conceive how the Earth is every where habitable;
and how passengers can travel quite round it; for since every thing
tends toward the center of the Earth, we are to conceive that point as
being the lowest, and not to carry our idea of downwards any farther.
Those that are diametrically opposite to us being as much upon the
upper part of the Earth as we are, there being no such thing in nature
as one place being higher than another, but as it is at a greater
distance from the center of the Earth, let it be in what country soever.
We have now done with all the circles of the sphere, and it may be
observed, that the _Equinoctial_, the _Ecliptic_, and the _Horizon_,
with their Secundaries and parallels, are all alike; and altering their
position, may be made to serve for one another. Thus, if the _Poles of
the World_ be brought into the _Zenith_ and _Nadir_, the _Equinoctial_
will coincide with the _Horizon_, the _Meridians_ will be the same
with the _Vertical Circles_, and the parallels of _Declination_ will
be the parallels of _Altitude_. After the same manner, if shifting
the position, we bring the _Ecliptic_ to coincide with the _Horizon_,
the circles of _Longitude_ will be the _Vertical Circles_, and the
parallels of _Latitude_ and _Altitude_ will coincide.
The horizon and the equator may be either parallel, perpendicular, or
oblique to each other.
[Sidenote: _Parallel Sphere._]
15. A _Parallel Sphere_ is that position where the equator coincides
with the horizon, and consequently the poles of the world are in the
Zenith and Nadir: The inhabitants of this sphere (if there be any) are
those who live under the poles of the world.
[Sidenote: _Right Sphere._]
16. A _Right_ or _Direct Sphere_ is that position where the equator is
perpendicular to the horizon, the inhabitants whereof are those who
live under the equinoctial.
[Sidenote: _Oblique Sphere._]
17. An _Oblique Sphere_ is when the equinoctial and the horizon make
oblique angles with each other, which every where happens but under the
equator and the poles.
[Sidenote: _Diurnal_ and _Nocturnal Arch_.]
The arch of any parallel or declination, which stands above the horizon
is called the _Diurnal Arch_; and the remaining part of it, which is
below the horizon, is called the _Nocturnal Arch_.
That point of the equinoctial which comes to the (Eastern/Western)
part of the horizon with any point of the Heavens, is called the
(Ascension/Descension) of that point, counted from the beginning of ♈;
and if it be in a right sphere, the ascension or descension is called
right; but if it be an oblique sphere it is called an oblique ascension
or descension. So that,
[Sidenote: _Right Ascension._]
18. The _Right Ascension_ of the _Sun_, _Moon_, or any _Star, &c._ is
an arch of the equator contained betwixt the beginning of ♈, and that
point of the equinoctial which rises with them in a _Right Sphere_, or
which comes to the meridian with them in an oblique sphere.
[Sidenote: _Oblique Ascension._]
19. _Oblique Ascension_, or _Descension_, is an arch of the equinoctial
intercepted between the beginning of ♈, and that _Point_ of the
_Equator_ which rises or sets with any point in the Heavens in an
oblique sphere.
[Sidenote: _Ascensional Difference._]
20. _Ascensional Difference_, is the difference betwixt the right and
oblique ascension or descension, and shews how long the Sun rises or
sets before or after the hour of six.